Plant water relations of thornscrub shrub species, north-eastern Mexico

Abstract

As an approach to understand how diurnal and seasonal plant water potentials (Ψ) are related to soil water-content and evaporative demand components, the responses of six thornscrub shrubs species (Havardia pallens, Acacia rigidula, Eysenhardtia texana, Diospyros texana, Randia rhagocarpa, and Bernardia myricaefolia) of the north-eastern region of Mexico to drought stress were investigated during the course of 1 year. All study species showed the typical diurnal pattern of variation in Ψ. That is, Ψ decreased gradually from predawn (Ψ_pd) maximal values to reach minima at midday (Ψ_md) and began to recover in the late afternoon. On a diurnal basis and with adequate soil water-content (>0.20 kg kg⁻¹), diurnal Ψ values differed among shrub species and were negatively and significantly (p<0.001) correlated with air temperature (r=−0.741 to −0.883) and vapor pressure deficit (r=−0.750 to −0.817); in contrast, a positive and significant (p<0.001) relationship was found to exist with relative humidity (r=0.758–0.842). On a seasonal basis, during the wettest period (soil water-content>0.20 kg kg⁻¹), higher Ψ_pd (−0.10 MPa) and Ψ_md (−1.16 MPa) values were observed in R. rhagocarpa, whereas lower figures (−0.26 and −2.73 MPa, respectively) were detected in A. rigidula. On the other hand, during the driest period (soil water-content<0.15 kg kg⁻¹), Ψ_pd and Ψ_md values were below −7.3 MPa; i.e. when shrubs species faced severe water deficit. Soil water-content at different soil layers, monthly mean relative humidity and monthly precipitation were significantly correlated with both Ψ_pd (r=0.538–0.953; p<0.01) and Ψ_md (r=0.431–0.906; p<0.05). Average soil water-content in the 0–50 cm soil depth profile explained between 70% and 87% of the variation in Ψ_pd. Results have shown that when gravimetric soil water-content values were above 0.15 kg kg⁻¹, Ψ_pd values were high and constant; below this threshold value, Ψ declined gradually. Among all shrub species, A. rigidula appeared to be the most drought tolerant of the six species since during dry periods it tends to sustain significantly higher Ψ_pd in relation to B. myricaefolia. The remaining species showed an intermediate pattern. It is concluded that the ability of shrub species to cope with drought stress depends on the pattern of water uptake and the extent to control water loss through the transpirational flux.

Introduction

The main type of vegetation in north-eastern Mexico, known as the Tamaulipan thornscrub, is composed of diverse, dense and spiny shrubs and trees which are distinguished by a wide range of taxonomic groups exhibiting differences in growth patterns, leaf life-spans, textures, growth dynamics, and phenological development (Reid et al., 1990; McMurtry et al., 1996; Northup et al., 1996). This semi-arid shrubland, which covers about 200,000 km² including southern Texas and north-eastern Mexico (Johnston, 1963; Udvardy, 1975), is characterized by an average annual precipitation of 805 mm and the yearly potential evapo-transpiration is about 2200 mm. In this region, the thornscrub has been utilized as a forage source for domestic livestock and wildlife, fuelwood, charcoal, timber for construction, fences, herbs, medicine, agroforestry and reforestation practices in disturbed sites (Reid et al., 1990). In terms of productivity of the native vegetation, averaged biomass production has been estimated to be about 3.2 Mg ha⁻¹ year⁻¹ in dry weight (Villalón, 1989). Since water availability is the most limiting factor controlling shrub growth, survival, and distribution in dry climates (Kramer, 1983; Newton and Goodin, 1989), the great diversity of native shrub species in this region reflects the plasticity of how shrub species cope with seasonal water stress. Therefore, shrub plants have evolved key morphological and physiological traits suited for adaptation to environmental constraints, especially in drought-prone sites. The strategies include early leaf abscission, limited leaf area, an extensive and deeper root system, epidermal wax accumulation, reduction of water loss by stomatal closure and accumulation of organic and inorganic solutes (Newton et al., 1991). Although two pieces of evidence are available on plant water relations for shrub species of the north-eastern region of Mexico (Stienen et al., 1989; González et al., 2000), this region provides a rich opportunity to investigate ecophysiological aspects of native shrub species and their responses to changes in resource availability, particularly soil moisture content, in order to gain a better understanding of how to sustain and improve productivity. The objectives were to assess and quantify how diurnal and seasonal plant water potentials are related to soil water availability and evaporative demand components in six shrub species that grow in north-eastern Mexico.